6533b86dfe1ef96bd12c9491

RESEARCH PRODUCT

Experimental and theoretical study of line mixing in methane spectra. III. The Q branch of the Raman ν1 band

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The shape of the ν1 Raman Q branch of CH4 perturbed by Ar and He at room temperature has been studied. Stimulated Raman spectroscopy (SRS) experiments have been made in the 2915–2918 cm−1 spectral region for total pressures from 0.4 to 70 atm and mixtures of ≈5% CH4 with He and Ar. Analysis of the spectra demonstrates that the shape of the Q branch is significantly influenced by line mixing and much narrower than what is predicted by the addition of individual line profiles. For the first time, a model is proposed for the calculation and analysis of the effects of collisions on the considered spectra. In this approach, the rotational part of the relaxation matrix is constructed, with no adjustable parameter, starting from semiclassical state-to-state rates. Two empirical constants which account for the shift and broadening of the branch due to vibrational effects are introduced and their values are determined from fits of measured spectra. Comparisons between measurements and results computed with and wit...